This invention relates to a circuit for protecting a DC control element from short circuits. More particularly, the invention relates to a protection circuit for a control element having at least one transistor which is driven by timing pulses obtained by comparison of a control voltage with a voltage having a triangular wave form.
In one known commercially available equipment, the load current of a DC control element is monitored by a current-measuring device in the load circuit; current in the DC control element is stopped as soon as the load current exceeds a predetermined maximum value. This does not result in short-circuit protection for all cases, since not every short-circuit current in the circuit flows through the current-measuring device in the load circuit. Thus, in DC control elements having a bridge ciruit, the currents caused by a short circuit in a bridge arm, for instance, are not detected. In addition, shorts to ground cannot be reliably recognized. Immediate disconnection after an overcurrent results in the DC control element being definitely put out of operation after only one occurrence of an overcurrent. The system is, thus, sensitive to interference.
An overload protection device for individual transistors is described in the literature reference etz-b, vol. 30 (1978), no. 26, page 1065. There the transistor to be protected is monitored for overcurrent. If desaturation occurs, the transistor is switched off. A newly arriving "on" pulse is coupled via a capacitor to the transistor and switches the latter on again.
It is an object of the present invention to provide a short-circuit protection device of the type mentioned at the outset in which short circuits are detected with certainty and in which the DC control element is shut off only when a short circuit prevails over an extended period of time. Short circuits are to be understood in the following to include those shorts to ground which lead to overcurrents in the transistors of the DC control element.